This invention relates to fluorescent lamps which are adapted as direct replacements for existing incandescent lamps. More specifically, this invention concerns fluorescent lamps wherein ionization is induced by a transformer which is partially contained within the lamp envelope.
The incandescent lamp is the primary luminary for household and residential lighting. This lamp generally includes an incandescent filament within a predetermined nonoxidizing atmosphere which is contained within a teardrop shaped envelope and mounted, for example, within an Edison type base which is screwed into a permanent fixture or into a movable socket.
Despite their widespread use, incandescent lamps are relatively inefficient, producing only 15-17 lumens per watt of input power and have relatively short, unpredictable service lives. Fluorescent lamps, which have efficiencies as high as 80 lumens per watt, provide an attractive alternative to incandescent lighting. Conventional fluorescent lamps, however, require a long tubular envelope which, together with the need for auxiliary ballasting equipment, has somewhat limited their acceptance in the home lighting market. Increased residential use of fluorescent illumination, with attendant savings of energy, can be achieved from the development of fluorescent lamps which are directly compatible with existing sockets and incandescent lamp fixtures.
The electric lamp technology has long sought electric discharge devices which produce visible light for general illumination purposes without the utilization of electrodes as the footpoints of a glow or arc discharge. Although the concept of electrodeless discharge lamps is very old, such lamps have always included the concept of coupling electrical energy into an hermetically sealed gas-containing envelope by means of a ferromagnetic or air core transformer to avoid the use of electrodes. Such devices have never proved practical or commercially feasible, because it has been impossible to achieve any reasonable efficiency of light emission due to the utilization of iron or air core transformers because of core losses, among other factors.
It has been proposed in the prior art to excite electrodeless gaseous discharge lamps using electromagnetic induction to transfer electric energy into the discharge vessel. Experiments along this line disclose that heretofore such means have been highly impracticable. If an air core transformer is utilized, the inefficiency of the coupling procedure required to achieve a reasonable power input to the gaseous discharge results in a loss of power by radiation which is prohibitive, and which may be dangerous. Accordingly, such devices have never been successfully operated for useful periods at any reasonable efficiency.
Another alternative that has been proposed in the prior art is the utilization of iron or ferromagnetic cores. Such cores, however, may be utilized only at very low frequencies in order that eddy current heating of the iron does not cause core failure. Utilizing alternating current, it is exceedingly difficult to operate an iron core transformer for the purpose of transferring energy of this nature at frequencies in excess of five or ten kilohertz. Based upon experimental and calculated results obtained in this laboratory, it has been determined that for an iron core transformer operating at 50 kHz, core power losses are in the range of approximately 80 to 90 percent. Accordingly, from the foregoing it may readily be appreciated that air core and iron core transformers are, from a practical point of view, inoperative at the high radio frequency levels that are necessary for efficient operation of gaseous discharge lamps in accord with this invention.
In my prior U.S. Pat. 3,500,118 and 3,521,120, I disclosed fluorescent lamps which utilize a magnetically induced radio frequency electric field to ionize a gaseous radiating medium. The elimination of discharge electrodes within these lamp envelopes substantially increases their life and allows lamp shapes which are more compatible with home lighting needs.
My U.S. Pat. No. 3,500,118, issued Mar. 10, 1970, describes an improved electrodeless fluorescent lamp having a radio frequency power supply. This structure, while useful, was bulky comprising a large tubular discharge ring, several ferrite cores, and a remotely mounted power supply which made it unsuitable for use in many industrial and residential applications.
My later U.S. Pat. No. 3,521,120, issued July 21, 1970, describes a more compact lamp. However, this lamp maintains a high frequency magnetic field in the air surrounding the envelope and thus sometimes constitutes an unacceptable source of electromagnetic radiation and interference.
Briefly stated, my concurrently filed patent application Ser. No. 642,142 discloses fluorescent lamps which may be constructed within a spherical or teardrop shaped structure typical of residential incandescent lamps. A toroidal magnetic core contained within the lamp envelope is excited with a radio frequency magnetic field. The core induces an electric discharge in gas contained within the lamp. Radiation from this gas excites a conventional lamp phosphor on the inner surface of the envelope and on the outer surface of the core to produce visible light.
The magnetic core of the lamps disclosed in application Ser. No. 642,142 is contained wholly within the lamp envelope under vacuum conditions. Despite the improved efficiency of those lamps, up to a quarter of the radio frequency input power is dissipated within the core and winding structures. To provide for efficient operation, the operating temperature of ferrite cores and lamp phosphors are, generally, restricted to the region below 125.degree.C. Thus, limited heat transfer from the transformer core is a major factor in determining the maximum light output available from thos lamps. Structures disclosed in my patent application Ser. No. 642,142, include metal rods for conducting heat through the lamp envelope so that it may be dissipated to the atmosphere.
The operation and efficiency of fluorescent lamps may be degraded if materials within the envelope act to contaminate the phosphors or to change the gas pressures. Many magnetic materials, which are otherwise suitable for use in these lamps, tend to outgas at lamp pressures and may evolve potentially damaging substances, for example oxygen and water vapor, into the envelope. In addition, many of these materials are generally unreceptive to the application of phosphor coatings. In accordance with the teachings of my concurrently filed patent application, Ser. No. 642,142, a thin glazing is applied to ferrite lamp components to improve the adherence of phosphors (which may then be applied using conventional Lehring processes) and to contain gases which may evolve from the cores.